1. Field of the Invention
The present invention relates to a gas sensor for mounting in a pipe defining a flow path in which a gas-to-be-measured such as an exhaust gas flows, and functions by introducing air as a reference gas from the outside.
2. Description of the Related Art
In the related art, various kinds of sensors such as an HC sensor or an NOx sensor are known as gas sensors for detecting the density of a specific gas component in a mixed gas. For example, as shown in FIG. 12, an oxygen sensor 501 comprising a detecting element 502 formed of an oxygen ion conductive solid-state electrolyte is known.
The detecting element 502 described above comprises an internal electrode 503 and an external electrode 504 formed respectively on the inner and outer surfaces of the element body of a cylindrical shape with a bottom wherein one end is closed by a solid-state electrolyte and the other end is left open. Between the internal electrode 503 and the external electrode 504, an electromotive force is generated depending on the difference in the concentration or partial pressure of oxygen between the internal space in the detecting element 502 formed in a cylindrical shape and the outside. For example, the concentration of oxygen in a gas-to-be-measured can be detected by inserting the closed end of the detecting element 502 into the interior of the pipe defining a flow path of a gas-to-be measured via a cylindrical main fitting 505 to allow the external electrode 504 to come into contact with a gas-to-be-measured and the internal electrode 503 with the air, and then detecting an electromotive force generated between the internal electrode 503 and the external electrode 504.
Introduction of air into the oxygen sensor may be carried out via a clearance formed in twisted core wires of a lead 507. However, in order to introduce a large amount of air, a method of forming an air hole 509 on the side surface of a cylindrical cover 506 and plugging it by a water repellant filter 508 having gas permeability formed, for example, of a fluorine resin in order to prevent water or the like from entering therein has been employed.
When such an oxygen sensor is used for detecting the density of oxygen contained in an exhaust gas at an elevated temperature, the water repellant filter 508 may be deteriorated in gas permeability by heat due to exposure to an elevated temperature after long term use.
In order to solve this problem, as stated in Japanese Patent Laid-Open No.54063/97, a method of forming an air hole on the sealing member provided on the side of the opened end of the cylindrical cover facing toward the outside of the pipe defining a flow path for a gas-to-be-measured, and inserting a bar-shaped filter through the air hole is considered. By disposing the filter in the sealing member, the sealing member is positioned at a distance from the flow path and thus heat is not easily transferred from the pipe defining a flow path. Consequently, the possibility that the filter is exposed to an elevated temperature may be reduced. However, such a bar-shaped filter has a problem in that high resistance is generated when a gas is passed through and thus introduction of the air may be insufficient.
Such a problem occurs not only in an oxygen sensor, but also in gas sensors such as the NOx sensor or the HC sensor that require introduction of air.
In view of the above, it is therefore an object of the present invention to provide a gas sensor which functions by introducing air from the outside, wherein the property of introducing air may be maintained even when used at an elevated temperature.
Accordingly, a gas sensor according to a general aspect of the present invention comprises a cover including a gas permeable sealing member having an air hole and a gas permeable water repellant filter for closing said air hole, wherein said water repellant filter is formed in a sheet-shape and mounted in said air hole via a cylindrical inserting member that can be inserted into said air hole.
A gas sensor according to a first aspect of the present invention comprises a detecting element having electrodes on first and the second surfaces of an oxygen ion conductive solid-state electrolyte, a main fitting having a fitting portion to be fitted into a mounting hole formed on a wall of a pipe defining a flow path for a gas-to-be-measured for holding said detecting element in such manner that said first surface is disposed via said mounting hole at an inner position of said pipe with respect to said fitting portion, a cylindrical cover of which one end is connected to an outer position of said pipe with respect to the fitting portion of said main fitting and the other end is provided with a cylindrical sealing member having an air hole for introducing air to said second surface on one end and a through hole through which a lead connected to both electrodes of said detecting element passes on the other end, and a water repellant filter having gas permeability for closing said air hole, characterized in that said water repellant filter is formed in a sheet shape and mounted on said air hole by means of a cylindrical inserting member that can be inserted in said air hole.
In the gas sensor according to the first aspect of the present invention, since the water repellant filter is formed in a sheet shape, preferably gas permeability can be obtained. In addition, since the water repellant filter is mounted on the sealing member and thus it is hardly affected by heat, the gas permeability can be maintained even when said gas sensor is exposed to heat from the outside.
According to a second aspect of the invention, in order to fix the water repellant filter described above to the air hole, the water repellant filter is preferably interposed between the outer peripheral surface of the cylindrical inserting member and the inner peripheral surface of said air hole so as to close the opened end of the cylindrical inserting member.
In the gas sensor in this arrangement according to the second aspect of the invention, the water repellant filter can be mounted to the air hole easily by pressing the water repellant filter into the air hole by means of a cylindrical inserting member. It is also possible to cover the opening of the cylindrical inserting member in advance, and then insert the cylindrical inserting member into the air hole.
In the structure according to the second aspect, the water repellant filter is interposed or directly brought into intimate contact between the inner peripheral surface of the air hole and the outer peripheral surface of the cylindrical inserting member so that the sealing property between them is establised. Since the opened end of at least one cylindrical inserting member is closed by the water repellant filter, it can prevent passage of water drops through the inside of the cylindrical inserting member, thereby realizing waterproof at the air hole of the sealing member.
Though it is preferable to interpose the water repellant filter between the outer peripheral surface of the cylindrical inserting member and the inner peripheral surface of the air hole because the effects as described above may be obtained as described thus far, there is a possibility that the water repellant filter is displaced. As a consequence, there is a possibility that sufficient waterproof property may not be established at the air hole.
According to a third aspect of the invention, the cylindrical inserting member is preferably provided on its outer peripheral surface at the portion that comes into contact with the water repellant filter with an engaging portion with which the water repellant filter engages when the water repellant filter is interposed between the portion of the outer peripheral surface of the cylindrical inserting member being in contact with the water repellant filter and the inner peripheral surface of the air hole.
In the gas sensor in this arrangement according to the third aspect of the invention, since the water repellant filter is positively retained by the engaging portion when being interposed between the outer peripheral surface of the cylindrical inserting member and the inner peripheral surface of the air hole, displacement caused by shrinkage of the water repellant filter itself under the environment of an elevated temperature, or expansion and shrinkage of the sealing member due to thermal cycle does not occur easily, thereby increasing reliability of the water proof property.
An engaging portion may be constructed as a through hole formed on the side surface of the cylindrical inserting member or a dented hole formed on the outer peripheral surface of the cylindrical inserting member. Since the sealing member is resilient, the water repellant filter is forced into the hole portion by being pressed by the sealing member and thus the water repellant filter is engaged.
An engaging portion may also be constructed as a projection protruding from the outer peripheral surface of the cylindrical inserting member outwardly. The water repellant filter is then forced into and engaged with the sealing member by being pressed by said projection.
In the structure according to the second and third aspects of the invention, it is preferable to arrange the water repellant filter in such manner that the edge thereof can be visually observed as in the fourth aspect of the invention. In this arrangement, an inspection can be made to determine whether or not the water repellant filter is stably interposed between the outer peripheral surface of the cylindrical inserting member and the inner peripheral surface of the air hole.
According to a fifth aspect of the invention, it is also possible to interpose the water repellant filter between the inner peripheral surface of the outer fitting member that can be fitted on the outer peripheral surface of the cylindrical inserting member and the outer peripheral surface of the cylindrical inserting member.
In the gas sensor according to the fifth aspect of the invention, for example, the water repellant filter can be disposed at the air hole by a procedure comprising the steps of fixing the water repellant filter on the cylindrical inserting member in advance (with the opened end of the cylindrical inserting member closed), and inserting this cylindrical inserting member into the air hole of the sealing member. In other words, the cylindrical inserting member and the water repellant filter can be handled as a unit. Therefore, for example, an inspection can be made to determine whether or not the water repellant filter is positively closing the opened end of the cylindrical inserting member in the stage where the cylindrical inserting member is assembled as a unit. As a consequence, nonconformity of waterproofing can be discovered in the early stage of assembly, thereby reducing production cost.
According to a sixth aspect of the invention, it is preferable that the water repellant filter is interposed between the outer peripheral surface of opened end of the cylindrical inserting member closed by said water repellant filter and the inner peripheral surface of the outer fitting member
In this arrangement, since the water repellant filter is clamped by the outer peripheral surface of the opened end to be closed, the area of the water repellant filter required to close the opened end of the cylindrical inserting member (and thus the air hole of the sealing member) may be reduced, thereby reducing the cost required for manufacturing the water repellant filter.
According to a seventh aspect of the invention, it is preferable that the outer peripheral surface of the cylindrical inserting member is provided with a limiting portion for limiting the movement of the outer fitting member along the axis of the cylindrical inserting member. According to the gas sensor in this arrangement, positioning of the outer fitting member on the outer peripheral surface of the cylindrical inserting member can be made easily, thereby facilitating assembly. Displacement of the water repellant filter can be prevented as well.
The limiting portion may be constructed as a projection (for example, a flange-shaped projection) protruding outwardly from the outer peripheral surface of the cylindrical inserting member, or as a shoulder in which the thickness of the cylindrical inserting member changes in the direction of the axis.
According to an eighth aspect of the invention, it is preferable that the outer fitting member and the cylindrical inserting member (not limited to the limiting portion) are directly and partly in contact with respect to each other. In the portion where both members are in direct contact with respect to each other (in other words, the portion where the water repellant filer is not interposed), both members can be joined without damaging the water repellant filter by providing a projection and a depression that can be fitted with respect to each other or by crimping, thereby ensuring that the outer fitting member is fixed to the cylindrical inserting member.
In the gas sensor according to the fifth to eighth aspects of the invention, fixing the water repellant filer to the cylindrical inserting member in such a manner that the edge thereof can be visually observed as in the ninth aspect of the invention enables the user to inspect whether or not the water repellant filer is stably interposed between the outer peripheral surface of the cylindrical inserting member and the inner peripheral surface of the outer fitting portion.
According to a tenth aspect of the invention, the part of the outer peripheral surface of the cylindrical inserting member coming into contact with the water repellant filter is preferably roughened. This arrangement ensures that the water repellant filter is more positively retained on the outer peripheral surface of the cylindrical inserting member.
According to an eleventh aspect of the invention, it is preferable to provide the portion of the outer peripheral surface of the cylindrical inserting member that comes into contact with the water repellant filter with a tapered portion that tapers in a direction away from the opened end that is closed by the water repellent filter. In this arrangement, the water repellant filter is pulled in the direction away from the opened end to be closed, whereby the water repellant filter closes the opened end of the cylindrical inserting member more reliably.
According to a twelfth aspect of the invention, the opened end of the cylindrical inserting member closed by the water repellant filter preferably projects from the opened end of the air hole facing toward the outer portion of said gas sensor. In this arrangement, even when a liquid such as water or oil is trapped in the vicinity of the opened end of the air hole facing toward the outside of said gas sensor, the water repellant filter that closes the open end of the cylindrical inserting member is prevented from being covered by a liquid.
The water repellant filter may be constructed by a porous fiber structure formed, for example, of polytetrafluoroethylene (PTFE). However, when manufacturing such a filter into a sheet-shape by drawing it in a certain direction, the resultant fiber structure exhibits a property to be more likely to stretch in the direction orthogonal to that direction. Such a structure may have a property of shrinking in one direction when heated. In addition, when interposing the water repellant filter between the inner peripheral surface of the air hole or the outer fitting member and the outer peripheral surface of the cylindrical inserting member, the filter may be fixed in the state of being stretched. For example, when inserting the filter into the air hole in the state of covering the cylindrical inserting member, a frictional force between the filter and the inner surface of the air hole stretches the filter. However since the stretching property of the filter differs depending on the direction, it stretches in the stretching direction and accordingly it shrinks in the direction orthogonal to the stretching direction, and thus it is difficult to insert uniformly. There may be a case where the length of the filter is long enough for being interposed in one direction, but is too short to be interposed in the other direction thereby making it difficult to be stably interpose the filter.
Therefore, according to a thirteenth aspect of the present invention, in the case where the water repellant filter has an anisotropic property in the extent of shrinkage when heated or when inserted, the filter may be formed longer in the direction in which it is apt to shrink in advance. In this arrangement, even when shrinkage occurs upon heating, the water repellant filter can be retained between the inner peripheral surface of the air hole or the outer fitting member and the cylindrical inserting member. As a detailed configuration of the filter, an oval shaped filter having a longer axis in the shrinking direction and a shorter axis in the stretching direction as stated in the fourteenth aspect of the invention may be employed. The oval figure resists exhibiting a directional property as described above, so that the water repellant filter is uniformly held between the cylindrical inserting member and the sealing member. The configuration of the filter is not limited to an oval figure, and the configuration having a longer axis and a shorter axis such as a rectangular shape may also be employed.
As stated in a fifteenth aspect of the invention, it is preferable to form the air hole coaxially with the sealing member, and to form the cylindrical inserting member with a material harder than the sealing member.
In the gas sensor having such an arrangement according to the fifteenth embodiment of the invention, unevenness of a stress generated inside the sealing member by a crimping operation may be prevented, thereby improving the sealing property due to the sealing member. Any material may be employed for the cylindrical inserting member as long as it is capable of resisting an external force, and metals such as SUS 304 or a resin such as PTFE are preferable.
According to a sixteenth aspect of the present invention, it is preferable to provide a flange that engages with the sealing member on the outer peripheral surface of the cylindrical inserting member.
According to a gas sensor in this arrangement as set forth in the sixteenth aspect of the invention, the flange provided on the outer peripheral surface of the cylindrical inserting member preferably engages the sealing member, thereby resisting detachment of the cylindrical inserting member from the air hole.
In the gas sensor according to the sixteenth aspect of the invention, the flange of the cylindrical inserting member may be constructed to engage any portion of the sealing member. For example, though the flange may be constructed so as to engage the outer peripheral surface around the opening of the air hole, a part of the cylindrical inserting member may overhang out of the air hole and may result in limiting the layout of other parts by narrowing the space therearound.
Therefore, the flange is preferably constructed to engage the engaging portion formed in the air hole according to a seventeenth aspect of the invention.
In the gas sensor according to the seventeenth aspect, since the flange engages with the engaging portion formed in the air hole, it does not overhang outside the air hole and thus the space can be used effectively.
According to an eighteenth aspect of the invention, the flange preferably has a notch. In this arrangement, the water repellant filter engages the notch, and thus displacement of the water repellant filter can be prevented. Though the flange may hide the edge of the water repellant filter and thus visual observation cannot be made, it is also possible to enable visual observation through the notch.
According to a nineteenth aspect of the invention, a water repellant filter having an oil repellant property is preferably used. In this arrangement, the water repellant filter prevents oil from attaching thereon, thereby ensuring gas permeability of the air hole.
According to a twentieth aspect of the invention, in order to positively prevent passage of liquid such as water drops or oil through the air hole, two pieces of the water repellant filters are preferably provided in the direction of the axis of the air hole.
According to a twenty-first aspect of the invention, forming the outer surface of the sealing member facing toward the outside of said gas sensor into a convex shape preferably prevents the outer surface from trapping or being covered by liquid.
According to a twenty-second aspect of the invention, the convex shape of the outer surface of the sealing member is preferably formed so as to be highest at the opened end of the air hole facing toward the outside of said gas sensor to prevent gas permeability from becoming deteriorated due to more efficient attachment of liquid on the water repellant filter.
According to a twenty-third aspect of the invention, the water repellant filter may be adhered and fixed on its periphery directly to the opened end of the cylindrical inserting member. In this case, though troublesome work to adhere the outer peripheral edge of the water repellant filter to the opened end of the cylindrical inserting member is required, the amount of the costly water repellant filters to be used may be reduced.